The preparation of aromatic and aliphatic-aromatic carbonic acid esters (carbonates) by transesterification starting from aliphatic carbonic acid esters and aromatic hydroxy compounds is known in principle. It is an equilibrium reaction in which the position of equilibrium is shifted almost completely towards the aliphatically substituted carbonates. It is therefore comparatively simple to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, in order to carry out the reaction in the opposite direction, in the direction towards aromatic carbonates, it is necessary effectively to shift the equilibrium, which is positioned very unfavourably, to the side of the aromatic carbonates, it being necessary to use not only very active catalysts but also suitable procedures.
It is known to carry out such equilibrium reactions in columns and thus advantageously shift them towards the formation of the desired product (e.g. U. Block, Chem.-Ing. Techn. 49, 151 (1977); DE-OS 38 09 417; B. Schleper, B. Gutsche, J. Wnuck and L. Jeromin, Chem.-Ing.-Techn. 62, 226 (1990); Ullmanns Encyclopädie der techn. Chemie, 4th Edition, Vol. 3; p. 375 ff. 1973).
In the known processes, the transesterification is therefore also preferably carried out continuously as a countercurrent transesterification in one or more reaction columns.
EP-A 0 461 274 describes a continuous transesterification process for the preparation of aromatic carbonates in one or in a plurality of multistage columns connected in series, wherein dialkyl carbonates or alkylaryl carbonates are reacted with phenols and the readily volatile products, namely the reaction alcohols and dialkyl carbonates, are removed at the head of the columns and the high-boiling products, such as, for example, diaryl carbonates, are removed at the sump of the columns. Particular process measures which allow the transesterification to be carried out more advantageously by adapting the apparatus and procedures to the above-described special problems of this transesterification are not described, however. In particular, there is no indication for the person skilled in the art whether the column used for the transesterification is preferably constructed with or without a concentrating part.
In a construction with a concentrating part, the vaporous mixture removed at the head of the column contains substantially the dialkyl carbonate used in excess in the reaction and the corresponding alcohol formed in the reaction, and the condensation of the mixture takes place at a temperature below the evaporation temperature of the dialkyl carbonate at the pressure prevailing in the reaction, with the result that the heat of condensation that is obtained can be dissipated only at a low temperature level compared with the temperature in the reaction zone. If the transesterification column is constructed without a concentrating part, then the vapour mixture removed at the head of the column, and accordingly also the distillate from the column, contains not inconsiderable amounts of the aromatic hydroxy compounds used for the reaction and, optionally, also of even higher-boiling components.
When the dialkyl carbonate used in excess is recovered, for example by separation from the reaction alcohol by distillation, amounts of higher-boiling components, such as, for example, the aromatic hydroxy compound, lead to an additional separation problem or at least to an increased temperature level during the supply of heat.
DE-A 42 26 756 describes a two-stage process for the preparation of diaryl carbonates, in which in a first stage an aromatic hydroxy compound is reacted with a dialkyl carbonate in a countercurrent transesterification, there being used a countercurrent transesterification column which can be constructed both with and without a concentrating part. In the case of an arrangement with a concentrating part, the temperature level in the condensation is comparatively low, so that the heat of condensation formed thereby cannot be used economically for operating other process sections. In the case of an arrangement without a concentrating part, the distillate, as well as containing the dialkyl carbonate and the alcohol formed in the reaction, additionally also contains a high proportion of the aromatic hydroxy compound. During recovery of the dialkyl carbonate, this leads to an increased temperature level in the region of the column sump, which makes the supply of energy at that point more difficult.
DE-A 42 26 755 describes a process for the preparation of diaryl carbonates in two reaction columns which are coupled with one another in terms of energy and materials, wherein an aromatic hydroxy compound and a dialkyl carbonate are reacted in the first stage, and the alkylaryl carbonate formed thereby is converted into the diaryl carbonate in the second stage either by transesterification with the aromatic hydroxy compound or by disproportionation. However, a problem with this process is that, owing to the integration of the process in terms of energy and materials, the reaction conditions for the formation of the alkylaryl or diaryl carbonate cannot be chosen optimally because they are determined by the almost identical pressure prevailing in the two steps. Accordingly, the described process does not bring any advantages in terms of energy integration.
EP-A 781 760 describes a continuous process for the preparation of aromatic carbonates by reacting a dialkyl carbonate with an aromatic hydroxy compound in the presence of a catalyst and continuously removing the aromatic carbonate formed in the reaction, the alcoholic secondary products, the dialkyl carbonate and the aromatic hydroxy compound, the dialkyl carbonate and the aromatic hydroxy compound being fed back into the reaction again. Although the described process steps are effective as regards the reaction procedure in terms of a high space-time yield and as regards working-up in terms of an as efficient separating sequence as possible, the process does not exhibit any possibilities for integration of the reaction and the working-up steps in terms of energy.
WO-A 2006/001256 describes a process in which an aromatic hydroxy compound is reacted with a dialkyl carbonate in the presence of a catalyst, as well as a technical device suitable therefor. Here too, no reference points are given for energy integration.
Without appropriately efficient energy integration, the energy consumption of the processes described hereinbefore is known to be high, which in turn raises questions about the advantageousness of the phosgene-free preparation of aryl carbonates from an ecological and economic point of view.
WO-A 2004/016577 describes a process for the preparation of aromatic carbonates from dialkyl carbonate and an aromatic hydroxy compound in the presence of a catalyst in a plurality of separate, series-connected reaction zones of a reactor arrangement, wherein the heat of condensation that is formed in the condensation of the vapour stream of the last reaction zone is used to heat the liquid stream introduced into the first reaction zone. However, this process has the disadvantage that the reactor arrangement is complex. In addition, the energy integration of this process is worthy of improvement.